This invention relates to the recovery of wood fibre from fibreboard, including M.D.F. (Medium Density Fibreboard), chipboard, particle board, hardboard, and the like fibre-based board, (hereinafter referred to generically as xe2x80x9ccompressed fibreboardxe2x80x9d).
Particle board, commonly called wood chipboard in the UK, is made from small chips or particles of wood coated with a synthetic resin glue (normally urea-formaldehyde) and pressed into sheets.
From the mid 1940""s chipboard revolutionized the furniture industry, in that it made it possible for furniture to be made from a large flat panel without joints. At the present day, almost all manufactured furniture utilizes chipboard in place of natural wood and because of the suitability of chipboard to have melamine, decorative paper veneers and postform worktops applied in the factory, a massive industry has been created.
With the introduction of Medium Density Fibreboard (M.D.F.) in the 1970""s, the necessity to cover the edges of the board was overcome. Furniture makers found they could now rout the edges so their pieces looked like furniture manufactured from solid wood. M.D.F. is an ideal substrate for machining and finishing. Modem use of this material is now extended even to, the production of architraves and wall skirting used in most houses built today.
It is estimated that the total consumption figure for the UK is around 2.5 million cubic meters of chipboard products and around 0.5 million cubic meters of M.D.F. products per year. At an average density of 750 kg/cubic meter this represents an annual consumption of around 3.75 million tonnes. Similar consumption figures would also apply to each of the major European Countries.
Even at this time of cost conscious production methods, utilizing computer controlled cutting and profiling machinery, an average furniture manufacturer will waste in the region of 5% of the raw materials used. Such wastage is in the form of off-cuts, trimmings, spoiled panels and dust produced in the sawing and milling operations. On the basis of this, the total amount of such waste material entering the UK waste stream is at least 350,000 tonnes per year.
Major difficulties have however been encountered in the recovery of processed wood products such as chipboard and M.D.F. Such waste material contains around 8% urea-formaldehyde resin and is often covered with a bonded melamine or paper veneer. Thus the use of conventional recovery equipment such as shredders or chippers is not practical since the additional items such as the decorative coating would contaminate the potential recovered product. Further difficulties exist if such recovered material is used for animal bedding, through possible respiratory problems associated with the fine dust and the formaldehyde content.
Further, conventional shredding equipment only shreds the material, thus the in-line grain chips, demanded by the end user for chipboard or paper uses, would be torn and rendered useless.
A small proportion of the M.D.F. and chipboard waste is, nonetheless, used in the animal bedding market and a small proportion is also used as a Waste to Energy fuel. However, at the present time most of the chipboard and M.D.F. waste generated by the furniture industry in the UK enters the general controlled waste stream which is predominantly directed to landfill disposal. The cost of such landfill disposal has increased dramatically in recent years and is expected to increase further.
In contemplating any feasible and cost effective method that could be utilized to recover the fibrous content of such wood based products, it is important to understand the methods of the manufacture of both chipboard and M.D.F. Production of both chipboard and M.D.F. involves the use of a synthetic resin binder and adhesive. The synthetic resin glue normally used in both particle board and M.D.F. production is a urea-formaldehyde solution containing a small proportion of paraffin wax.
Chips of wood are coated with the resin and a matt of coated wood chips is formed on a stainless steel caul or platen which is then fed to a heavy duty press. The rate of closure of the press as well as the actual pressure and controlled high temperature applied influence both the physical and mechanical properties of the finished board.
In the case of the production of M.D.F., the wood fibres are coated with urea-formaldehyde resin after a defibration process and similarly pressed into aboard, but at much higher pressure and increased thickness of the fibrous matt, to achieve the extremely high density required for the finished board material.
An alternative method of manufacture can be used for the production of hardboard and some fibreboards. In this method, the board is made without the use of a resin adhesive and the manufacture relies on the ligno-cellulosic quality of the wood fibre. The basic strength and adhesion is obtained from the felting together of the fibres themselves and from their own adhesive properties.
An object of the invention is to provide a process and apparatus for treating waste fibreboard to enable the wood fibre thereof to be recycled.
A further object of the invention is to enable the separation of NI.D.F. and chipboard into their various components, i.e. woodfibres, resin and coatings, without destroying or deforming the actual fibres or contaminating the recovered fibre with melamine or other coatings. According to this invention there is provided a method of recovering wood fibres from compressed fibreboard scrap, involving the steps
a) admitting the scrap into a container;
b) subjecting the scrap in the container to steam;
c) screening the scrap to separate wood fibres from the remainder of the scrap.
Preferably waste material is treated without pre-processing. Waste in the form of off-cuts, trimmings, spoiled panels etc. may be steam treated as received from the furniture manufacturers or other suppliers.
Preferably the scrap is agitated in the container while being subjected to steam. By so agitating the scrap, preferably gently, the scrap opens up, allowing steam to penetrate into the interior of the pieces of scrap, removing the adhesives such as urea-formaldehyde, without any significant damage to the wood fibres.
The action of the steam on the fibreboard breaks down the resin binding agent which then forms a liquid effluent. Advantageously this is drained off and can be recycled for use again. By using the steam treatment of the waste, the quantity of effluent liquid produced is low in proportion to the volume of material treated.
Preferably the steam contains additives such as wetting agents.
Preferably the steam treatment is carried out under pressure. Advantageously an increase in pressure over atmospheric pressure of greater than 1 p.s.i., preferably in the range of 3 to 10 p.s.i. is used. Although in certain circumstances a higher pressure may be utilized. Advantageously the steam treatment is carried out for at least 30 minutes.
Preferably liquid effluent is extracted from the chamber during the steam treatment process. Alternatively the liquid effluent may be extracted at the end of the steam treatment. Advantageously any excess steam and fumes are extracted from the steam chamber at the end of the steam treatment. This ensures that no fumes are emitted during operation and that, once the steam treatment is completed, the environment in the steam chamber will be safe.
Preferably subsequent to the subjection of the scrap to steam under a pressure greater than atmospheric pressure, the scrap is subjected to a low pressure, preferably at least 1 p.s.i. less than atmospheric pressure, preferably in the range of 3 to 10 p.s.i. less than atmospheric pressure, although in certain circumstances, particularly where certain types of phenolic resin are used, a lower pressure may be desirable.
Preferably the scrap is agitated while being subjected to said low pressure.
Such application to the scrap of low pressure carries out two major functions, the first being that the break-up of the scrap material is increased, as the pieces of scrap tend to further expand under the action of the low pressure. Again this preferably aided by the continuous agitation of the scrap within the container exposing the interior of the scrap and separating the chips from surface decoration of the scrap, such as melamine.
The second function of the low pressure is that it allows fumes to be withdrawn from the container, so that noxious components thereof may be removed, such as by scrubbing.
If desired scrap may be subjected to repeated steam pressure and low pressure treatment stages, conveniently said stages under these circumstances being of shorter duration.
Preferably the method involves transferring the treated scrap from the container to a screening device, conveniently afforded by a rotary trommel.
Advantageously the rotating action of the trommel breaks the steam treated waste pieces down. The separated woodfibre and/or woodchips pass through appropriate mesh gauges of screen and are then collected and any material such as melamine, Formica etc. that is not broken down bypasses the screen plates and is fed out for collection in a suitable receptacle.
Preferably the trommel pass rate is slow enough to enable the steam treated waste pieces to be sufficiently broken down by the rotating action of the trommel.
Preferably the process of the invention comprises the further step of drying the collected woodfibre and/or woodchips.
Preferably the process comprises the further step of processing the collected woodfibre and/or woodchips for re-use.
Preferably the woodfibre is further processed for use in paper, cardboard or board manufacturing.
Preferably the woodchips are further processed, for example by defibration, for use in paper manufacturing. Alternatively, the woodchips may be further processed for use in chipboard.
According to this invention there is provided an apparatus for the recovery of wood fibre from compressed fibreboard scrap comprising
a) a container wherein the scrap may be loaded;
b) a steam generator to apply steam to the container; and
c) means to agitate the scrap in the container while steam is applied thereto.
Preferably the container is afforded by a drum, and said means to agitate the scrap in the container comprises means to rotate the drum, conveniently for short periods in opposite directions.
Advantageously the means to agitate the scrap in, the container also comprises vanes or ribs mounted in the container, such as extending helically within the container to xe2x80x9ctumblexe2x80x9d the scrap within the container as the container rotates.
Preferably the container is mounted within a sealed vessel comprising inlet means for the admission of steam, and outlet means for the withdrawal of moisture from the container.
Preferably the steam chamber is fitted with a liquid drainage sump in the base. Preferably the apparatus comprises extraction means connected to the top of the steam chamber and operative, upon the completion of a steaming cycle, to evacuate excess steam and any liquid prior to the opening of the chamber doors. Alternatively the steam and fume evacuation may be carried out by a separate evacuation system.
Sealing of the chamber and evacuation at completion of the steam cycle ensures that no fumes are emitted during operation and that, once opened, the chamber environment will be safe.
According to this invention there is also provided a waste recovery plant comprising apparatus as set out above, and screening apparatus to which treated scrap is fed from the container.
Preferably the screening apparatus comprises a rotary trommel screen. Advantageously the rotary trommel screen can be fitted with different screening surfaces and different size screen meshes. Alternatively, the screening may be carried out by the use of a vibrating table-type machine.
Preferably the plant further comprises drying means to dry the collected woodfibre and/or woodchips. Advantageously the drying means comprises a drying chamber.
Preferably the plant further comprises processing means to refine the collected woodfibre and/or woodchips. In one embodiment, the processing means may comprise a defibration plant to treat recovered woodfibre for use in the paper making industry.